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schema: introduce the `index:offset_of` method

The method is used to get the (potential) 1-based iterator-relative
position in the index of a tuple matching a key.

Closes #8204

@TarantoolBot document
Title: A new `index:offset_of` method introduced
Product: Tarantool
Since: 3.3.0

The method returns 0-based offset in the index of a first tuple
matching the provided key and iterator. The position is counted from
the beginning or end of the space depending on the iterator direction,
for example:

``` lua
-- index: {{1}, {3}}
index:offset_of({3}, {iterator = 'eq'})  -- returns 1: [1, <3>]
index:offset_of({3}, {iterator = 'req'}) -- returns 0: [<3>, 1]
```

In case there's no tuple matching the key and iterator in the index the
function returns the position a matching tuple would be placed at if
existed, for example:

``` lua
-- index: {{1}, {3}}
index:offset_of({2}, {iterator = 'eq'})  -- 1: [1, <2>, 3]
index:offset_of({4}, {iterator = 'req'}) -- 0: [<4>, 3, 1]
```

This works with any iterator:

``` lua
-- index: {{1}, {3}}
index:offset_of({0}, {iterator = 'ge'}) -- 0: [<1>, 3]
index:offset_of({1}, {iterator = 'lt'}) -- 2: [3, 1, <...>]

-- index: {{'b'}, {'bb'}, {'bc'}, {'c'}, {'cc'}}
index:offset_of({'b'},  {iterator = 'np'}) -- 3: [b, bb, bc, <c>, cc]
index:offset_of({'cc'}, {iterator = 'pp'}) -- 1: [cc, <c>, bc, bb, b]
```

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